WO1993017333A2

WO1993017333A2 - Carbon analyser

Info

Publication number: WO1993017333A2
Application number: PCT/GB1993/000353
Authority: WO
Inventors: Christopher George Perry
Original assignee: Fluid Dynamics (Sales) Limited
Priority date: 1992-02-19
Filing date: 1993-02-19
Publication date: 1993-09-02
Also published as: GB9203642D0; WO1993017333A3; AU3637893A

Abstract

An analyser for measuring carbon in a sample comprises a furnace (3), a reaction vessel (7), a dehumidifier (8), a detector (10), and means (1) for flowing the sample in a carrier gas through the furnace and/or the reactor vessel to produce carbon dioxide, which is passed via the dehumidifier to the detector, characterised in that a carrier gas flow control unit (2) is provided downstream of the furnace. The analyser may also comprise means (20) for providing an additional supply of carrier gas to the carbon dioxide passing through the detector, the additional supply of carrier gas acting to dilute the carbon dioxide.

Description

CARBON ANALYSER

The present invention relates to a carbon analyser for performing total carbon (TC), inorganic carbon (IC), and total organic carbon (TOC) analyses. Generally a single analyser, a TOC analyser, is used to perform all three of these measurements, and the invention will . hereinafter be described chiefly with respect to such a TOC analyser.

TOC analysers are used for testing the carbon content of water in order to establish its purity or its level of impurity. Such' TOC analysers are well known, and generally comprise a furnace in which the total carbon content of a sample is oxidised into carbon dioxide. Typically, the sample combustion products then pass through an inorganic carbon reaction vessel and a dehumidifier before reaching a non-dispersive infra-red (NDIR) detector which measures the carbon dioxide concentration. The total carbon (TC) content in a sample can readily be established from the carbon dioxide content by using a calibration curve. The TC content is the sum of the total organic carbon (TOC) content and the inorganic carbon (IC) content of the sample. In a separate step, the sample is first introduced into the IC reaction vessel where only the IC content is converted to carbon dioxide. After passing through the dehumidi ier, this is then measured by the NDIR detector. The TOC content is then obtained by subtracting the IC content from the TC content of the sample. In known TOC analysers, the combustion products are carried from the furnace, through the IC reaction vessel and dehumidifier and into the NDIR detector by means of a carrier gas which is usually oxygen or high purity air, and a carrier gas flow control unit is provided between the source of gas and the furnace, since the flow rate of the carrier gas is a determinant of the concentration of the combustion products passing through the detector.

The high purity air which is used as the carrier gas in such analysers is usually a composite of pure nitrogen and pure oxygen and typically should not contain more than 1 part per million (ppm) each of CO2 (carbon dioxide), CO (carbon monoxide) and HC (hydrocarbon). Carrier gas containing excessive impurities would impair the accuracy of the TOC measurement. The effect of impure carrier gas on the accuracy of measurements is greater when higher sensitivity measurements are being recorded.

The output of ^*the detector, which is proportional to the concentration of carbon dioxide in the gas stream, is passed to a data processing unit and can be displayed graphically, the total carbon content being proportional to the area under the peak of the graph. Thus the TC in a sample (or the IC content if that measurement is being performed) can easily be determined from a calibration curve prepared using standard solutions of known carbon content. The volume of the sample can be selected to ensure that the carbon content is not unmanageably large but that it is of sufficient size to produce a reasonably accurate measurement- Typically, the scale of the axes of the graph can be selected from a number of available ranges as appropriate, such that the peak height extends to within approximately fifty to ninety-five percant of the maximum range of the NDIR detector.

Whilst the currently available TOC analysers are reasonably satisfactory, there are various problems in their use, and in particular they can be difficult to calibrate and operate in such a way as to obtain reliable, accurate and reproducible results. We have now found that this problem can be mitigated or overcome by moving the gas flow control unit to a position downstream of the furnace. This ensures that the flow rate through the NDIR detector is constant and unaffected by changes in the flow rate which occur, for example, at the point when the sample is introduced into the furnace.

Accordingly, one aspect of the present invention provides an analyser for measuring carbon in a sample, which analyser comprises a furnace, a reaction vessel, a dehumidifier, a detector, and means for flowing the sample in a carrier gas through the furnace and/or the reactor vessel to produce carbon dioxide, which is passed via the dehumidifier to the detector, characterised in that a carrier gas flow control unit is provided downstream of the furnace.

Preferably, the gas flow control unit is provided as near as possible upstream of the detector, between the dehumidifier and the detector. Preferably the detector is an NDIR detector.

A further disadvantage of known TOC analysers is that if, for example the selected sample volume is too large, such that the peak height extends beyond the range of the NDIR detector, or is too small, such that the peak height is less than fifty percent of the range of the NDIR detector thus increasing the inaccuracy of the measurement, a different sample volume has to be selected and the scale range of the NDIR detector may need to be altered, and then the whole process of passing the sample through the furnace and the IC reaction vessel has to be repeated.

According to a second aspect of the present invention there is provided an analyser for measuring carbon in a sample, which analyser comprises a furnace, an IC reaction vessel, a dehumidifier, a detector, and means for flowing the sample in a carrier gas through the furnace and/or reaction vessel to produce carbon dioxide, which is passed via the dehumidifier to the detector, characterised in that the analyser further comprises means for providing an additional supply of carrier gas to the carbon dioxide passing through the detector, the additional supply of carrier gas acting to dilute the carbon dioxide.

Preferably, the additional supply of carrier gas is controlled by a mass flow control unit.

Thus, in the present invention the additional supply of carrier gas acts as a diluent to control the concentration of carbon in the gas passing through the detector.

Conventionally, a sample is introduced into the reaction vessel of a TOC analyser using a syringe and septum. It is possible 'to procure specially developed septa, needles and "septumless" injection valves for TOC or carbon analysers. However, these systems and components have a number of limitations, particularly if large diameter syringe needles have to be used, for example in the analysis of suspended solids.

According to a third aspect of the present invention there is provided sample introduction apparatus for introducing a sample into a reaction vessel by means of a needle, the apparatus comprising a valve and an annular gland providing access to the valve, the apparatus being connected to the reaction vessel, wherein the valve is positioned adjacent the gland such that when a sample is to be introduced into the reaction vessel, the needle is insertable through the gland and through the valve to introduce the sample into the vessel and the gland is adapted to seal around the needle. According to a further aspect of the invention there is provided a method of introducing a sample into a reaction vessel comprising the steps of: a) inserting a needle of a syringe containing the sample through a tightly fitting gland connected to the reaction vessel; b) opening a valve between the gland and the reaction vessel; c) inserting the needle through the valve; and d) releasing the sample through the needle into the reaction vessel.

This method and apparatus for sample introduction aims to solve the problems associated with conventional septum/syringe sample introduction.

The present invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:

Figure 1 shows schematically a prior art configuration of a TOC analyser;

Figure 2 shows schematically a TOC analyser in accordance with a first'aspect of the present invention;

Figure 3 shows schematically a TOC analyser in accordance with a second aspect of the present invention; and

Figure 4 shows apparatus for sample introduction into a reaction vessel in accordance with a third aspect of the present invention.

A number of the basic elements shown in the Figures are common to each of the TOC analysers shown, and like reference numerals will be used for like parts.

The prior art TOC shown in Figure 1 comprises a carrier gas supply 1 and a gas flow control unit 2 which controls the flow rate of carrier gas to a furnace 3. The sample to be measured is held in syringe 4. The outlet line 5 from the furnace is also connected to the IC reaction vessel.

An outlet line 6 from the IC reaction vessel 7 passes to a dehumidifier 8 and the dehumidifier is connected via a line 9 to a non-dispersive infra-red (NDIR) detector 10. The detector is connected to a data processing unit (DPU) 11 with suitable peripherals (not shown) such as a keyboard, a visual display unit and a printer.

A portion of the measured sample volume can then be injected by the syringe 4 into the furnace 3, through a septum 14.

The furnace 3 is packed with a catalyst and maintained at a very high temperature: in the region of 680^C so that when the sample enters the furnace the carbon in the sample is oxidised into carbon dioxide. All the carbon in the sample will be converted into carbon dioxide. The carrier gas is pure air and thus provides oxygen to assist the combustion process in the furnace, which also contains a catalyst. The carrier gas then carries the combustion products through the outlet line 5 to the IC reaction vessel 7.

The products are carried by the carrier gas from the reaction vessel 7 into line 6 and through the dehumidifier 8 which removes the water from the products. The dehumidifier unit may also include a halogen and acid scrubber (not shown) for treating the products before passing them to the NDIR detector 10 to measure the carbon dioxide content of the sample. This information is processed, stored and displayed as required. The output signal of the NDIR detector is displayed as peaks on a graph of concentration of carbon dioxide in the sample against time. The peak area is measured and processed by the DPU. The peak area is proportional to the total carbon (TC) concentration in the sample such that the TC content can readily be determined from a calibration curve preparedusing standard sample solutions of known carbon concentration.

The TC content is the sum of the total organic carbon (TOC) content and the inorganic carbon (IC) content, and therefore the IC content of the sample has now to be established. To do this, a controlled volume of the sample in syringe 4 is passed into the IC reaction vessel 7 through a septum 15. The IC reaction vessel 7 has carrier gas passing through the acid solution. When the sample contacts the acid, only the inorganic carbon (which is in the form of carbonates and hydrogen carbonates) in the sample is converted into carbon dioxide, since the acid will not aff ct the organic carbons. Thus the carrier gas takes the reaction products through the de amidifier 8 to the NDIR detector 10 where the carbon dioxj.de content which is proportional to the IC content of the sample can be measured and the information processed by the DPU 11.

The TOC content of the sample is then obtained by subtracting the IC content from the TC content.

The analyser may also provide a direct line 12 by which high purity air ca^'n be passed into an acidified sample held in container 13, such that the air sparges the sample to remove the IC, the total non-purgeable organic carbon content of the sample can then be measured directly.

As discussed in the introduction, there are a number of disadvantages which result from the arrangement shown in Figure 1. In particular, such a TOC analyser can be difficult to operate in such a way as to obtain reliable and reproducible results, particularly when injection volumes are changed to suit the particular range of the NDIR detector 10.

The first aspect of the present invention mitigates or overcomes this problem by placing the gas flow control unit 2 downstream of the furnace 3. In the particular embodiment shown in Figure 2, the gas flow control unit 2 is placed immediately upstream of the detector 10.

Figure 3 illustrates the second aspect of the present invention wherein an additional supply 20 of carrier gas passes through a second gas flow control unit 21 and via a line 22 which connects with the NDIR detector 17. The additional supply of -carrier gas, which may be taken directly from the carrier gas supply 1 or may be supplied from a separate source, acts as a diluent for the combustion and reaction products passing through the detector. Thus the concentration of carbon in the gas products passing through the detector can be controlled such that the peak height on the graph does not exceed the maximum range of the NDIR detector and thus go off the scale, necessitating remeasurement of a smaller volume of the sample liquid.

The second gas flow control unit 21 is preferably very sensitive such that the dilution of the gas products can be very accurately controlled. In a particularly preferred embodiment the two gas flow control units 2 and 21 are controlled by means of a computer such that the quantity of carrier gas "being passed through the system by gas flow control units 2 and 21 is known exactly, and the measurement of carbon concentration can be adjusted accordingly to give an accurate final reading of the sample.

Although the present invention has primarily been described in relation to apparatus functioning as a TOC analyser, clearly the invention also extends to a method of analysing the TOC content of a sample as employed by the apparatus described, wherein the flow rate of carrier gas is controlled at a point downstream of the furnace. The method may also comprise the step of diluting the combustion and reaction products by the addition of a controlled supplementary flow of carrier gas. A preferred method and apparatus for introduction of a sample into a reaction vessel 39 of a TOC analyser will now be described with reference to Figure 4.

A sample introduction valve comprises an annular needle guide and gland 31, made from polytetrafluoroethylene (PTFE) for example. The gland has an inside diameter slightly less than the outside diameter of a selected needle 34 of a syringe (not shown). The external end of the gland is tapered to guide the needle through the gland.

A fitting 32 holds the gland on a valve chamber 37 and when the fitting is tightened down on the gland it compresses the gland reducing its internal diameter.

A valve 33 is supported in the valve chamber adjacent the gland 31. The valve 33 may be in the form of a conventional plug or ball valve such that when the valve is in the open position (as shown), the valve orifice 38 is larger than the outside diameter of the syringe needle 34.

The operation of the sample introduction valve is as follows:- a) the valve 33 is initially in the closed position b) the needle 34 is pushed through the gland 31 ensuring that the fitting 32 is tightened down. c) The valve 33 is opened. No leakage occurs at the gland 31, because of the tight fit between the needle 34 and the gland 31. d) The needle is then pushed completely through the valve orifice 38. e) The sample in the syringe may now be dispensed into the reaction vessel 39. f) Withdrawal of the needle is a reversal of steps (a) to (d).

The operation of the sample introduction valve may be improved by automating the process. This requires the valve 33 to be fitted with an actuator (not shown) . The actuator then operates automatically (for example, electrically or pneumatically) when the needle is inserted into the gland 31. The insertion of the needle is sensed, for example, by a microswitch or proximity sensor installed between the gland 31 and the valve 33.

Alternatively, by using an auto-sampler where the movement and operation of the needle is motorised and moves according to programmed instructions, the actuator may be operated by a relay contact from the auto-sampler.

Using the above described sample introduction valve, much larger volumes of sample may be introduced into the reaction vessel than in prior art techniques. This can give rise to problems with the oxidation or reaction which takes place in vessel 39, as comparatively high pressures occur inside the reaction vessel 39. To obviate this, an expansion vessel 36 is fitted to the outlet of the reaction vessel. The expansion vessel preferably has a volume which is approximately 30 to 50% of the volume of the reaction vessel. This allows the reaction product to escape easily, so that pressure does not build up too excessively inside the reaction vessel.

Consequently, more repeatable and faster reactions occur with more regular and predictable peak shapes.

The sample introduction valve described may also be used in other laboratory analysis or sample introduction applications where, for example, a septum or injection valve would currently be employed: for example in chromatogr phy.

Claims

1. An analyser for measuring carbon in a sample, the analyser comprising a furnace, a reaction vessel, a dehumidifier, a detector, and means for flowing the sample in a carrier gas through the furnace and/or the reactor vessel to produce carbon dioxide, which is passed via the dehumidifier to the detector, characterised in that a carrier gas flow control unit is provided downstream of the furnace.

2. An analyser according to claim 1, wherein the gas flow control unit is provided as near as possible upstream of the detector, between the dehumidifier and the detector.

3. An analyser according to claim 1 or claim 2, wherein the analyser further comprises means for providing an additional supply of carrier gas to the carbon dioxide passing through the detector, the additional supply of carrier gas acting to dilute the carbon dioxide.

4. An analyser for measuring carbon in a sample, which analyser comprises a furnace, an IC reaction vessel, a dehumidifier, a detector, and means for flowing the sample in a carrier gas through the furnace and/or reaction vessel to produce carbon dioxide, which is passed via the dehumidifier to the detector, characterised in that the analyser further comprises means for providing an additional supply of carrier gas to the carbon dioxide passing through the detector, the additional supply of carrier gas acting to dilute the carbon dioxide.

5. An analyser according to claim 3 or claim 4, wherein the additional supply of carrier gas is controlled by a mass flow control unit.

6. An analyser according to any preceding claim wherein the detector is an NDIR detector.

7. Sample introduction apparatus for introducing a sample into a reaction vessel by means of a needle, the apparatus comprising a valve and an annular gland providing access to the valve, the apparatus being connected to the reaction vessel, wherein the valve is positioned adjacent the gland such that, when a sample is introduced into the reaction vessel, the needle is insertable through the gland and through the valve to introduce the sample into the vessel, and the gland is adapted to seal around the needle.

8. Apparatus according to claim 7, wherein the reaction vessel incorporates an expansion chamber.

9. Apparatus according to claim 7 or claim 8, further comprising an actuator for controlling the valve, the actuator being responsive to the needle being inserted through the glan -

10. A method of introducing a sample into a reaction vessel comprising the steps of: a) inserting a needle of a syringe containing the sample through a tightly fitting gland connected to the reaction vessel; b) opening a valve between the gland and the reaction vessel; c) inserting the needle through the valve; and d) releasing the sample through the needle into the reaction vessel-